The border-width property allows you to set the width of all
four borders with only one declaration.

You can declare one, two, three or four widths:

If you declare one width, all four borders will be that width.

If you declare two widths, the top and bottom borders will be the
first width, the right and left borders will be the second width.

If you declare three widths, the top border will be the first, the
right border the second, and the bottom and left borders the third width.

If you declare four widths, the order is top, right, bottom, left.

thin | medium |
thick

The keyword values of thin, medium, or thick can be
used to set the width of the border line. The exact thickness of the
border created by using these keyword values will be determined by your
computer/browser combination.

length

The length value can be in any of the following seven units.
Use the abbreviation.